A polyamide-imide resin is excellent in heat resistance and mechanical properties, and has been used in various fields mainly in electric and electronic industries. However, in recent years, for the purpose of reducing the burden on the environment, the dissolving performance in a general-purpose solvent, such as diethylene glycol monoethyl ether acetate (EDGA) has been required. In addition, in view of heat resistance and mechanical properties, a polyamide-imide resin is expected to be used in the fields where transparency of the cured products, such as a liquid crystal display and an LED display, is required. In these fields, optical transparency from visible to ultraviolet range (approximately 300 nm) is required.
As the polyamide-imide resin dissolving in a general-purpose solvent, for example, a polyamide-imide resin obtained by reacting an isocyanurate type polyisocyanate (a1) of an isocyanate having an aliphatic structure with a trimellitic anhydride is disclosed (for example, refer to PTL 1). However, a cured product obtained by using the polyamide-imide resin disclosed in PTL 1 has insufficient transparency. For example, in the measurement of light transmittance, the optical transparency of the cured product in an ultraviolet range (approximately 300 nm) is not sufficient.
Meanwhile, it is known that a polyamide-imide resin obtained by reacting an isocyanurate type polyisocyanate (a1) synthesized from an isocyanate having an aliphatic structure with a tricarboxylic anhydride (a2) having an aliphatic structure is combined with a curable resin while maintaining the solubility in a general-purpose solvent, and further cured, so as to provide a cured product (cured film) having excellent transparency (refer to PTL 2). However, a curable resin composition obtaining by combining the polyamide-imide resin with a curable resin or a reaction diluent tends to have insufficient storage stability and short usable life and to have insufficient treatability.